Interference and noise are the main signal impairments affecting receiver performance in code division multiple access (CDMA) systems. Linear equalization receivers compensate for interference arising from dispersive channels (inter-symbol and/or inter-code), and compensate for noise coloration caused by the receive filter. Generalized Rake receivers perform symbol-level processing, while chip equalizer (CE) receivers perform chip-level processing, but they are equivalent examples of linear equalization receivers. A good introduction to G-Rake processing appears in G. E. Bottomley, T. Ottosson, and Y.-P. E. Wang, “A generalized RAKE receiver for interference suppression,” IEEE J. Select. Areas Commun., vol. 18, pp. 1536-1545, Aug. 2000.
As a generalization, linear equalization receivers set processing delays on estimated path delays, to collect desired signal energy, and set additional processing delays off-path, at positions good for characterizing interfering signal energy. Interference due to dispersion and noise are the main sources of performance degradation for the scenarios considered prior to release 7 of the Wideband-CDMA (WCDMA) standard (Release 7).
Release 7 introduces multiple-input multiple-output (MIMO) schemes, along with higher order modulation. Performance requirements for higher signal to noise ratios (SNR) than previously considered were also introduced. The increased complexities and more stringent signal quality requirements translate into requirements for more precise modulation and demodulation processes than were required for previous standard releases. Similar performance and precision improvements are being driven in other wireless standards, too, as increasingly complex modulation/demodulation schemes are adopted to achieve higher data rates.
The increased precision requirements for signal processing leads to a new source of interference that may be referred to as “timing offset.” For example, when multiple transmit (TX) antennas are used at a base station, each transmission chain has some inherent delay due to the physical components involved (cabling, filtering, etc.). If the base station transceiver does not compensate for the difference in delay between TX chains, the transmitted signals will be received with some time misalignment. Misalignment can be a significant issue if the transmissions are to be combined coherently at the receiver. Similarly, when multiple receive (RX) antennas are used, differences in the RX chains, such as filter group delay differences, can cause an effective time misalignment in the respective received signals. Again, this misalignment becomes a potentially significant performance issue in coherent combining.
Even in the absence of misalignment in parallel transmit/receive chains, timing offset issues arise from the coarseness of the timing grids used by many receivers for reporting the path delays of a multiple-component received signal. For example, a delay reporting grid on the order of one-half to one-quarter of a chip may be used to identify path delays in a typical WCDMA receiver. However, research has shown that delays must be tracked to within as little as one sixty-fourth of a chip to avoid performance degradation.
Various techniques are known for improved delay estimation. For example, sophisticated multipath delay estimation is taught in the commonly owned U.S. Pat. No. 6,839,378 B1 to Sourour, et al. In the '378 patent, one embodiment of delay estimation processes paths in ray strength order, so that delay estimation is improved by subtracting out the influence of stronger paths for estimating the delays of the weaker paths. Further, the commonly owned U.S. Pat. No. 6,674,815 B2 to Zangi teaches exemplary techniques for tracking fractionally-spaced fading radio channels, for symbol-spaced estimation processing.
Still further, various techniques are known for improving Rake finger placement in view of the potential coarseness of the delay reporting grid. For example, different placement grids for path searching and Rake finger placement are used in the commonly-owned and co-pending U.S. patent application identified by application Ser. No. 10/653,679, which is published as US 2005/0047485 A1. In the '679 application, an independent timing grid of potentially finer resolution is used to place Rake fingers on and around signal path delays reported using a potentially coarser timing grid. The use of independent search and placement grids allows for some “tuning” of finger placement.
However, to the extent that path delays are identified and corresponding processing delays are placed according to timing grids limited to practical timing resolutions, one may expect timing offset issues potentially to be present in any practical receiver. Correspondingly, one technique for managing the potential performance problems arising from timing offset appears in the commonly-owned and co-pending U.S. patent application identified by application Ser. No. 11/219,183 and published as US 2006/0268962 A1. In the '183 application, a Rake receiver switches between one-finger and multi-finger modes, for example, based on determining which mode offers a better signal quality measure. The '183 application teaches that single-finger despreading yields better performance in the absence of appreciable timing offset between an estimated and an actual channel delay, while multi-finger despreading is better in the presence of appreciable timing offset.